The invention relates to a high-pressure discharge lamp comprising:
a light-transmitting lamp vessel which is closed in a gastight manner and which is provided with connection points; PA1 a first electrode and a second electrode which are arranged opposite each other and which are connected to respective connection points, and an ionizable filling in the lamp vessel; PA1 a light-transmitting outer envelope around the lamp vessel provided with a lamp cap; PA1 a first and a second external contact at the lamp cap for connecting the lamp to a supply device; PA1 a first and a second current conductor connecting, respectively, the first and the second electrode via respective connection points to, respectively, the first and the second contact; PA1 an ignition electrode between the first and the second electrode; PA1 a voltage-pulse generator in the outer envelope for generating a voltage pulse on the ignition electrode, which generator is connected to the ignition electrode.
Such a high-pressure discharge lamp is known from EP-B-0 477 621 (=U.S. Pat. No. 5,233,273).
In the known lamp, four current conductors enter the outer envelope from the lamp cap which is in open communication with the space in the envelope. An outer envelope which is not sealed from the environment in a gastight manner, however, is objectionable because it does not permit the temperature of the lamp vessel to be optimally regulated and because corrosion-sensitive components of the lamp, such as the parts of the current conductors entering the lamp vessel, are not protected by an inert medium in the outer envelope. On the other hand, it is difficult from the point of view of construction to introduce more than two current conductors into a closed outer envelope through a wall thereof.
A proper temperature control of the lamp vessel is more important as the pressures required in the lamp vessel are higher. An increase of the pressure in the lamp vessel generally leads to an improvement of the luminous efficacy of the lamp. On the other hand, in particular an increase of the noble gas pressure in the lamp vessel leads to an increase of the ignition voltage of the discharge lamp.
For a wide use of lamps having an improved efficacy, it is important to have the disposal of a high-pressure discharge lamp which can suitably be used with on an existing lamp holder.
In the case of a high-pressure discharge lamp known from said U.S. Pat. No. 5,233,273, the primary winding of a transformer in the outer bulb is connected by means of two separate conductors to an ignition circuit in the lamp cap. The secondary winding of this transformer is connected to the ignition electrode. The disadvantage of an ignition circuit arranged in the lamp and/or lamp cap is, inter alia, the high temperatures to which the circuit is exposed. This is important, in particular, for capacitive and semiconductor components which are used in the ignition circuit. A further disadvantage resides in that the realization of a sufficiently high ignition energy requires, in particular, a capacitive component which is relatively voluminous.
U.S. Pat. No. 4,910,437 discloses a high-pressure discharge lamp comprising an outer envelope which is closed in a gastight manner as well as an external ignition electrode which is present around the discharge vessel. A first and a second current conductor and a conductor connected to the ignition electrode are introduced into the outer envelope in a gastight manner. The voltage pulse on the ignition electrode is generated outside the lamp. In accordance with internationally accepted regulations, high-pressure discharge lamps for general lighting applications should ignite reliably at a limited ignition voltage applied to the lamp cap. In the case of a standardized lamp cap of the type E27 and E40, the permissible ignition voltage is 3 kV and 5 kV, respectively. Under specific conditions, the ignition by means of such a high ignition voltage pulse may involve the occurrence of large currents through the contact points. If higher voltage pulses are applied, use must be made of special lamp caps and associated lamp holders. The use of special lamp caps has the important drawback that the lamps used cannot be freely exchanged with existing lamps and they cannot be used on existing lamp holders. Nor is it possible to use these types of lamps as retrofit lamps in existing installations.
On the one hand, the permissible ignition voltage puts limitations on the aim to improve the luminous efficacy of high-pressure discharge lamps, while, on the other hand, it is attractive for general safety purposes if a high-pressure discharge lamp can be ignited with a substantially reduced ignition voltage pulse at otherwise equal lamp properties.